A strong demand has arisen in recent years for silver halide photographic materials, especially print photosensitive materials, which have increased photographic speed and which can be developed and processed more rapidly, and for the supply of high quality prints without the need for a high degree of skill. In order to respond to these demands the photographic performance of the photosensitive material must be improved and, at the same time, production stability must be achieved. Thus, any differences arising between production lots must be prevented and any changes in performance which may arise on long term storage after manufacture must be minimized.
Silver chlorobromides which are essentially free of silver iodide are used, mainly, in silver halide emulsions for print purposes from the viewpoint of increasing the development rate of these materials. Many attempts have been made in the past to increase the sensitivity of silver chlorobromides, but problems with low contrast and pressure sensitivity still remain.
For example, although emulsions which have been prepared by halogen conversion as disclosed in JP-B-50-36978 have increased photographic speed, it has been found that they are subject to pronounced desensitization when pressure is applied. (The term "JP-B" as used herein signifies an "examined Japanese patent publication".)
Furthermore, techniques involving the so-called lamination type emulsions, i.e., emulsions which have layers of different halogen composition within the silver halide grains, have been proposed in the specifications of, for example, JP-B-56-18939, JP-A-58-9137, JP-A-58-95736, JP-A-58-108533, JP-A-60-222844 and JP-A-60-222845. (The term "JP-A" as used herein signifies an "unexamined published Japanese patent application".) However, a softening of contrast in the toe part of the characteristic curve tends to occur in all of these cases, and there is a further disadvantage in that the materials tend to pressure desensitization.
On the other hand, differences between production lots and changes in performance (especially changes in photographic speed and fog level) on long term storage are a major problem in practice and these factors have been a major weakness, especially where high picture quality has been demanded, in recent years.
Such changes in performance are characteristic of the silver halide emulsions themselves, but at the same time it is thought that the performance of the sensitizing dyes which are used is also a major factor. That is to say, the amount of sensitizing dye adsorbed varies considerably on prolonged ageing of the emulsion's used for coating purposes during manufacture and on long term storage after manufacture.
This problem is especially acute when specified couplers, oils and organic solvents etc. are present in the same emulsion layer. That is to say, the sensitizing dye which should be adsorbed on the silver halide is gradually desorbed with the aging of the emulsions which are being used for coating purposes and with long term ageing after coating and drying.
In general, reported attempts to improve upon the differences which arise between production lots and to improve long term storage properties have included the addition of water soluble bromides (JP-A-52-151026), the addition of iridium salts (JP-A-54-23520), hardening agent selection (JP-A-60-202436 and JP-A-61-123834), the addition of super-sensitizing agents (JP-A-61-203447), and improvement of the method used to add the spectrally sensitizing dyes (JP-A-58-7629).
Moreover, it is suggested in JP-A-60-225147 that improvement of spectral sensitivity and ageing properties on storage, and improvement in respect of differences between emulsion lots, can be achieved by using silver chlorobromides which have (100) surfaces and (111) surfaces.
However, when these methods are used either individually or in combination it is very difficult to achieve improvements which actually provide satisfactory photographic performance, which is to say which provide satisfactory differences between production lots and long term ageing storage properties without loss of photographic speed, contrast and picture quality, etc.